1. Field of the Invention
The present invention relates to new photoresists, particularly photoresists that can crosslink after a development step, typically through thermal treatment. Resists of the invention are particularly useful to provide thermal flow coverage of semiconductor contact holes.
2. Background
Photoresists are photosensitive films used for transfer of images to a substrate. A coating layer of a photoresist is formed on a substrate and the photoresist layer is then exposed through a photomask to a source of activating radiation. The photomask has areas that are opaque to activating radiation and other areas that are transparent to activating radiation. Exposure to activating radiation provides a photoinduced chemical transformation of the photoresist coating to thereby transfer the pattern of the photomask to the photoresist coated substrate. Following exposure, the photoresist is developed to provide a relief image that permits selective processing of a substrate.
A photoresist can be either positive-acting or negative-acting. For most negative-acting photoresists, those coating layer portions that are exposed to activating radiation polymerize or crosslink in a reaction between a photoactive compound and polymerizable reagents of the photoresist composition. Consequently, the exposed coating portions are rendered less soluble in a developer solution than unexposed portions. For a positive-acting photoresist, exposed portions are rendered more soluble in a developer solution while areas not exposed remain comparatively less developer soluble.
Chemically-amplified-type resists have been increasingly employed, particularly for formation of sub-micron images and other high performance applications. Such photoresists may be negative-acting or positive-acting and generally include many crosslinking events (in the case of a negative-acting resist) or deprotection reactions (in the case of a positive-acting resist) per unit of photogenerated acid. In the case of positive chemically-amplified resists, certain cationic photo-initiators have been used to induce cleavage of certain xe2x80x9cblockingxe2x80x9d groups pendant from a photoresist binder, or cleavage of certain groups that comprise a photoresist binder backbone. See, for example, U.S. Pat. Nos. 5,075,199; 4,968,581; 4,883,740; 4,810,613; and 4,491,628, and Canadian Patent Application 2,001,384. Upon cleavage of the blocking group through exposure of a coating layer of such a resist, a polar functional group is formed, e.g., carboxyl or imide, which results in different solubility characteristics in exposed and unexposed areas of the resist coating layer. See also R. D. Allen et al., Proceedings of SPIE, 2724:334-343 (1996); and P. Trefonas et al. Proceedings of the 11th International Conference on Photopolymers (Soc. Of Plastics Engineers), pp 44-58 (Oct. 6, 1997).
Microelectronic devices frequently have multiple metal interconnection or conductive layers that are each separated by interposed insulating (dielectric) layers. The multiple conductive layers are connected using contact hole or via holes through the dielectric layers. See, generally, S. Sze, VLSI Technology (2nd ed., New York, McGraw-Hill, 1988), for a discussion of semiconductor device fabrication techniques.
I have now found improved compositions and methods for the fabrication of microelectronic devices. In particular, compositions and methods of the invention provide for a controlled flow of resist into device contact (via) holes during a post-exposure, post-development hard-bake step.
Resists of the invention are positive-acting and contain one or more components that are preferably substantially stable (i.e. no substantial crosslinking) during: 1) soft-bake, pre-exposure thermal treatment to remove solvent carrier of the applied resist, and 2) post-exposure, pre-development thermal treatment to promote or enhance the acid-promoted reaction in exposed regions (typically a de-blocking reaction). However, resists of the invention will crosslink during a post-development more stringent thermal treatment (thermal flow hard-bake step). By such selective crosslinking, the thermal flow rate of the applied resist into the contact holes can be controlled to within a desired range.
I have found that the use of such a thermal flow hard-bake when processing a contact hole resist can enable obtaining smaller critical dimensions than possible without post-development hard-bake processing. The hard bake (e.g. at least about 120xc2x0 C., more typically at least about 130xc2x0 C. or 140xc2x0 C., suitably from about 130xc2x0 C. to about 140xc2x0 C. to about 180xc2x0 C. or about 190xc2x0 C.) allows the resist to flow after development. However, in the absence of post-development crosslinking, the hard bake can produce too fast of a flow rate, which can limit resolution of formed features.
One preferred resist for use in accordance with the invention contains a photoactive component (typically a photoacid generator) and a resin with acetal and/or ketal moieties. The term xe2x80x9cacetalxe2x80x9d as used herein is inclusive of both acetal and ketal moieties, unless otherwise indicated. During a stringent post-development thermal flow hard-bake step, the resin can crosslink, typically by a transacetalation reaction. The hard-bake thermal treatment will cause flow of the resist as desired into a contact hole feature over which the resist has been applied, while the resist crosslinking will restrict the resist flow rate to a desired rate. At resist flow, the resist resin typically is above its Tg.
Suitable resist components that contain acetal groups that will react (crosslink) during a post-development hard-bake can be provided by a variety of routes. For instance, a vinyl ether (e.g. t-butyl vinyl ether) can be reacted with a hydroxy moiety such as phenolic xe2x80x94OH group to provide an acetal that will undergo reaction (particularly transacetalation) during a post-development hard-bake. Thus a polymer or copolymer containing phenolic units, such as a poly(vinylphenol) polymer or compolymer, can be reacted with a vinyl ether to provide the thermally reactive acetal moieties.
A variety of other resist systems can be employed in accordance with the invention provided one or more components of the resist can undergo crosslinking during a stringent hardbake step, but remain substantially stable (i.e. no substantial crosslinking) during prior thermal processing (i.e. pre-exposure soft bake and post-exposure, pre-development bake). For example, resists can employed that contain a resin that can contains ester groups (e.g. t-butyl ester groups) that can undergo crosslinking, such as by a transacetalation reaction.
Resists of the invention will typically contain separate components or functionalities that will be photoacid-labile and will be reactive upon exposure and any post-exposure, pre-development thermal treatment. Preferred photoacid-labile groups include acetal groups that are more reactive to photoacid-induced deblocking than the moieties that will crosslink during a post-development, hard-bake step. For instance, a resist resin can be employed that has both primary or second acetal groups and a tertiary acetal, or a primary acetal and a second or tertiary acetal. Without being bound by theory, the more branched acetal (i.e. secondary or tertiary) will more preferentially undergo transacetalization (crosslinking) at hard-bake temperatures, relative to a less substituted (i.e. primary or secondary) acetal, which less-substituted acetals will more preferentially de-block in the presence of photoacid after exposure and prior to development. See Scheme 1 below.
Resists of the invention also may contain a thermal acid generator, which is substantially stable to temperatures of a soft-bake step or a post-exposure, pre-development heat treatment, but can be activated to generate acid during more stringent temperatures of a post-development hard-bake step. The thermally generated acid then can promote crosslinking between resist component(s). However, in at least certain aspects of the invention, use of a thermal acid generator will be less preferred to avoid degradation of the resist during storage prior to use.
Resins used in resists of the invention can include a variety of units, including aromatic groups e.g. phenolic groups; cyano groups such as provide by polymerization of acrylonitrile or methacrylonitrile; and the like.
Polymers of the invention also may be substantially, essentially or completely free of phenyl or other aromatic groups, particularly for short wavelength imaging applications, such as 193 nm, 157 nm and other sub-200 nm wavelength exposures where aromatic groups can absorb excessive exposure radiation. Preferably such polymers have less than about 5 mole percent aromatic groups, based on the total polymer, more preferably less than about 3, 2, 1, 0.5, or 0.1 mole percent aromatic groups. Particularly preferred polymers for such wavelength imaging will be completely free of aromatic groups.
References herein to xe2x80x9ccrosslinkingxe2x80x9d or other similar term are intended to refer to essentially any covalent linkage between polymer chains or sites.
References herein that a polymer or other component xe2x80x9cdoes not undergo substantial crosslinkingxe2x80x9d or other similar phrase indicates that less than 20 mole percent of crosslinkable groups (i.e. crosslinkable upon subsequent more stringent post-development hard-bake) of the crosslinkable polymer or other component do not react upon exposure to a stated temperature for 60 seconds. Thus, for example, references herein that a polymer does not substantially crosslink at a post-exposure, pre-development bake of 120xc2x0 C. indicates that less than about 20 mole percent of crosslinkable polymer units (i.e. acetal or other units that can crosslink during the subsequent more stringent hard-bake) will crosslink during a 60 second exposure to the 120xc2x0 C. pre-development bake.
The invention also provides methods for forming relief images, including methods for forming a highly resolved relief image, and processing of contact (via) holes in microelectronic devices. The invention further provides articles of manufacture comprising substrates such as a microelectronic wafer with or without one or more contact (via) holes. Other aspects of the invention are disclosed infra.